The present invention relates to a light emitting diode module as a compact light source for a lighting unit incorporated in a contact image sensor type of image reader. More specifically, this invention relates to a light emitting diode module having the configuration capable of preventing destruction by static electricity.
In a contact image sensor type of image reader used, for instance, in a facsimile terminal equipment, a copying machine, or a scanner, a light emitting diode (described as LED hereinafter) is used as a compact light source for a linear lighting unit to emit light in a linear state to a surface of a document. An example of the linear lighting unit based on the conventional technology is disclosed in Japanese Patent Laid-open Publication No. 8-163320. As shown in FIG. 1, the linear lighting unit described in this publication incorporates therein a light conductor 14 within a white casing 12 for a linear lighting unit 10, and an LED module 16 comprising a plurality of LEDs is attached to a package having a form like a flat plate at one edge of the light conductor 14.
A full-color image reader often uses, as a linear lighting unit thereof, an LED module in which LEDS for three colors of red, green, and blue (R, G, B) are sequentially lighted up. Generally the LED module is a 4-terminal module in which a cathode and an anode of each LED are connected to individual terminals respectively and at the same time the anode and cathode are commonly connected to the respective common terminals.
FIG. 2 shows a connection diagram of a 4-terminal module. In the figure, designated at the reference numeral 20 is a red LED, at 22 a green LED, and at 24 a blue LED, and anodes of the LEDs are connected to terminals 26, 28, 30 respectively, while cathodes of the LEDs are commonly connected to a terminal 32.
Typically the red LED 20 has a conductive substrate, and a cathode or anode electrode is provided on a rear surface of the substrate, namely on a rear surface of the chip, while the anode or cathode electrode is provided on a top surface of the chip. The green LED and blue LED have an insulating substrate, and in many cases an node electrode and a cathode electrode are provided on a top surface of the chip. Because of the configuration described above, the connection with the anode connected to a common terminal as shown in FIG. 2 is implemented as shown in FIG. 3 in actual wiring.
In FIG. 3, packaged in a flat plate-formed package 34 (indicated by the dotted line) are lead frames 36, 38, 40 having individual terminals 26, 28, 30 shown in FIG. 2 at one edges respectively and a lead frame 42 having a common terminal 32 at one edge thereof. The red LED 20 is provided on the lead frame 36, the greed LED 22 on the lead frame 38, and the blue LED 24 on the lead frame 40.
The red LED 20 has an anode electrode 20A provided on a top surface of the chip and a cathode electrode (not shown) on a rear surface of the chip; the green LED 22 has an anode electrode 22A and a cathode electrode 22K both provided on a top surface of the chip; and the blue LED 22 has an anode electrode 24A and a cathode electrode 24K both provided on a top surface of the chip.
A cathode electrode of the read LED 20 is directly connected to the lead frame 36; the cathode electrode 22K of the green LED 22 is connected via a bonding wire 48 to the lead frame 38; and the cathode electrode 24K of the blue LED 24 is connected via a bonding wire 50 to the read frame 40.
The anode electrode 20A of the red LED 20, anode electrode 22A of the green LED 22, and anode electrode 24A of the blue LED 24 are connected via bonding wires 52, 54, and 56 respectively to the common lead frame 42.
It is generally known that an LED is disadvantageously vulnerable to a surge voltage caused by discharge of static electricity. When the terminal is connected, the LED may be destructed by a current higher than the rated one flowing in the reverse direction even for a very short period of time. It is generally recognized that especially the green LED and blue LED based on the conventional technology are more vulnerable to destruction caused by static electricity as compared to the red LED. For the purpose to prevent destruction by a surge voltage, load of a high voltage to an LED due to electrostatic discharge, generally a Zener diode is connected in parallel as shown in FIG. 4 to the LED or a Zener diode is provided on a substrate to which the LED module is connected to prevent a high voltage due to electrostatic discharge from being loaded to the LED.
However, when it is tried to incorporate a Zener diode in an LED module, a number of steps for packaging the chip not directly relating to the light emitting function into the module increases, which in turn causes disadvantageous problems such as the difficulty in size reduction of the module and increase in the production cost. Further, when a Zener diode is to be packaged on a substrate, the LED module may directly be affected by electrostatic discharge in the state where the terminal is connected until the LED module is packaged on the substrate, and it is required to take appropriate countermeasures for suppressing generation of static electricity in the peripheral environment.
It is an object of the present invention to provide a compact LED module capable of preventing destruction due to static electricity without using a Zener diode and requiring few countermeasures against static electricity in the peripheral environment.
In a first aspect of the present invention, the light emitting diode as a compact light source for a lighting unit has first, second, and third terminals; first and second light emitting diodes connected between the first and second terminals and also connected in inverse parallel to each other; and a third light emitting diode connected between the first or second terminal and the third terminal.
In this LED module, it is preferable that the first light emitting diode is a green light emitting diode, the second light emitting diode is a blue light emitting diode, and the third light emitting diode is a red light emitting diode.
In a second aspect of the present invention, the light emitting diode as a compact light source for a lighting unit has first, second, and third terminals; first and second light emitting diodes connected between the first and second terminals and also connected in parallelism to each other; and third and fourth light emitting diodes connected between the second and third terminals and also connected in inverse parallel to each other.
In this LED module, it is preferable that the first light emitting diode is a green light emitting diode, the third light emitting diode is a blue light emitting diode, and the second and fourth light emitting diodes are red light emitting diodes.